Reservoir fluids (liquids (such as water or oil) and gas) are found in geological reservoirs wherein they are contained at a high pressure (relative to ambient atmospheric pressure), and usually also at an elevated temperature (relative to ambient atmospheric temperature). At such pressures, the gas is dissolved in the liquid such that the reservoir fluid initially exists as a single-phase fluid, but the reservoir fluid will release dissolved gas to form a two-phase fluid with separate gas and liquid components if the reservoir fluid has its initial pressure sufficiently reduced towards ambient atmospheric pressure. Also, the initial relatively high temperature of the reservoir fluid results in volumetric contraction of a given mass of fluid as it cools towards ambient atmospheric temperature if withdrawn from the well.
When hydrocarbon exploration wells, for example, are drilled and hydrocarbon fluids are found, a well fluid test is usually performed. This test usually involves flowing the well fluid to surface, mutually separating the oil and gas in a separator, separately measuring the oil and gas flow rates, and then flaring the products (or transporting the products elsewhere for use or safe disposal).
It is also desirable to take samples of the reservoir fluid for chemical and physical analysis. Such samples of reservoir fluid are collected as early as possible in the life of a reservoir, and are analysed in specialist laboratories. The information which this provides is particularly-vital in the planning and development of hydrocarbon fields and for assessing their viability and monitoring their performance.
There are two ways of collecting these samples:
1. Bottom Hole Sampling of the fluid directly from the reservoir, and PA1 2. Surface Recombination Sampling of the fluid at the surface.
In Bottom Hole Sampling (BHS) a special sampling tool is run into the well to trap a sample of the reservoir fluid present in the well bore. Provided the well pressure at the sampling depth is above the "Bubble Point Pressure" of the reservoir fluid, all the gas will be dissolved in the liquid, and the sample will be a single-phase fluid representative of the reservoir fluid, i.e. an aliquot.
Surface Recombination Sampling (SRS) involves collecting separate oil and gas samples from the surface production facility (e.g. from the gas/liquid separator). These samples are recombined in the correct proportions at the analytical laboratory to create a composite fluid which is intended to be representative of the reservoir fluid, die a re-formed aliquot.
Several BHS tools are currently available commercially, which function by a common principle of operation. A typical BHS tool is run into the well to trap a sample of reservoir fluid at the required depth by controlled opening of an internal chamber to admit reservoir fluid, followed by sealing of the sample-holding chamber after admission of a predetermined volume of fluid. The tool is then retrieved from the well and the sample is transferred from the tool to a sample bottle for shipment to the analytical laboratory. As the tool in retrieved from the well, its temperature drops and the fluid sample shrinks causing the sample pressure to drop. This pressure drop occurs because the sample-holding chamber within the typical BES tool has a fixed volume after the sample is trapped and because the sample temperature is uncontrolled. Usually the sample pressure falls below the Bubble Point Pressure, allowing gas to break out of solution. This means the sample is now in two phases, a liquid phase and a gas phase, instead of in single-phase form as it was before the pressure dropped. In order successfully to transfer the sample from the tool to the sample bottle, it is necessary to pre-pressurise the sample sufficiently to force the free gas back into solution, recreating a single-phase sample. This recombination is a lengthy procedure and thus expensive.
The temperature change which the sample experiences and the resultant pressure change may also cause the precipitation of compounds previously dissolved in the well fluid, some of which cannot be re-dissolved by re-pressurisation. The absence of these compounds in the re-formed aliquot renders certain analyses meaningless.
A means by which a well fluid sample could be collected, retrieved and transferred in single-phase form, without a pressure-induced phase change, would a mitigate these problems. Not only would time spent recombining a two-phase sample back to single phase be saved, but pressure-sensitive compounds would remain dissolved, allowing more accurate analyses to be performed on the sample. One such means is described in our co-pending European Patent Application EP-A-0515495, which utilises pressurisation of the sample to maintain the sample in single-phase form.
In more general terms, it is also desirable to retrieve a sample whose temperature is close to its original temperature.